Colorado's ferocious Waldo Canyon wildfire has spread to an area of 67 sq km (26 sq miles), threatening more than 20,000 homes and other buildings. More than 1,000 firefighters are trying to douse the flames that have burned some luxury homes to their foundations and forced thousands to flee.

Overnight on Wednesday, television images showed flames shooting up the length of Blodgett Peak ridge, which overlooks the air force academy. The flames at one point blew past the fire lines, a spokeswoman for the Colorado Springs fire department said.

Neighbourhoods north and west of the city were shrouded in smoke. Some were deserted after authorities expanded their evacuation orders, blocking off main roads with patrol cars.

"We're the last stop to Armageddon," a server at a Starbucks close to the evacuation zone joked. "Every time they are told to evacuate they stop in here at the window for a caramel frappuccino."

More than 32,000 people were ordered out of their homes, with more than 300 of them being accommodated in five Red Cross shelters.

Others, like Mandy Osterbuhr in the Pleasant Valley neighbourhood, were told to be ready to leave at short notice.

"It's nerve wracking, just so scary," Osterbuhr said, carrying a plastic tub of toys out to the car.

She said she did not plan on staying until the official evacuation order. Thick black smoke was already pouring down from the hills, giving her daughter, Camille, nine, coughing fits and a nose bleed, and leaving a deposit of ash on the step.

The White House announced that Barack Obama would visit the area on Friday to thank firefighters for their efforts and to see the damage.

For most of those evacuated, it would be days before they knew the fate of their homes. But first reports suggested the destruction in some neighbourhoods was severe, said Rob Dyerberg, a fire information officer. A number of homes were burned to their cement foundations, he said. Others got off relatively lightly.

The prospect of losing everything weighed heavily on people forced out. Mary Taggs left her flat for a Red Cross shelter with just one small suitcase. "Sure, I've got insurance. There's a lot of sentiment," she said.

The decision on what to save left Osterbuhr just this side of panic. An four-wheel drive was packed with clothes and things of sentimental value. "I don't want to leave any of the kids' toys behind," she said. "You know, just in case."

Further down the street, her neighbours were making similar calculations, stacking suitcases by the front door, or ferrying documents and photo albums to the car.

The fires, which started on Saturday, broke through containment barriers on Tuesday night, fed by strong winds and a record heatwave.

It is the most serious of 15 wildfires across Colorado and dozens of wildfires across the American west because of its proximity to Colorado Springs, the state's second biggest city, and major tourist destination.

State officials have admitted there is little relief in sight because of the heatwave and high winds, deepening the sense of fear in neighbourhoods now threatened by the fire. Douglas Lingle, standing shirtless in his front yard, was reduced to helpless fury by the evacuation warning.

"I hope the whole thing burns to prove the government should have put out the fire a long time ago instead of pussy footing around. They let it get out of control," said Lingle, who divides his time between homes in Colorado and Texas. "If they can't put out a little bitty fire, how are they going to put out one that is in the whole town?"